1. Field of the Invention
The present invention relates generally to method and apparatus for determining material properties by microscopic scanning, and more particularly relates to determination of material properties including dopant profiles of materials, using capacitance-voltage techniques.
2. Background Art
Present day very large scale integrated (VLSI) circuits technology demands accurate knowledge of the spatial extent in three dimensions (3D) of active impurity dopants which have been incorporated into the discrete device elements. The devices are predominantly either bipolar or metal oxide semiconductor field effect (MOSFET) transistors, diodes, or capacitors. A typical device occupies an area of the order of 10 .mu.m.sup.2. The active region of such a device, where most current flows, is engineered by incorporating dopants, for example arsenic, boron, or phosphorous, in a concentration range of 10.sup.15 to 10.sup.20 cm.sup.-3. It is necessary to control the variation, or profile, of impurity dopants to a spatial resolution of 100 nm or less for high yield in manufacture and for reliability of the circuitry in the field. On the other hand, lack of precision related to the incorporation of impurity dopants can result in a proliferation of undesirable defects during later steps in the manufacturing process, and/or less than adequate device performance, or even device failure. Such high precision in the characterization of dopant profiles on a microscopic scale is, clearly, highly desirable for efficient device design. In order to achieve predictability in device behavior, one must be able to measure accurately the dopant profiles and feed this information back into the design cycle. However, heretofore it has been impossible to achieve this high precision, except in 1D, either in the design or manufacturing phases of VLSI components on the submicron scale.
Current techniques for quantitative measurement of dopant profiles are limited to high resolution in one dimension only. Such techniques include Secondary-Ion Mass Spectroscopy (SIMS), Spreading Resistance (SR), and macroscopic Capacitance-Voltage (C-V). For example, see S.M. Sze, "VLSI Technology" McGraw-Hill Book Co., New York (1983, see for example, Chapters 5 and 10). Other, non-quantitative methods exist, for example S.T. Ahn and W.A. Tiller, J. Electrochem. Soc. 135, 2370 (1988) and M.C. Roberts, K.J. Yellup and G.R. Booker, Institute of Physics Conference Series No. 76, No. 11 (Institute of Physics, London, 1985). In one dimension (1D) of the quantitative methods, only C-V satisfies the three criteria outlined above. However, as mentioned above, the data is provided in a single dimension only, and over a broad area.
For example, referring to FIG. 1, a cross section of a portion of semiconductor material 10 is shown, having an oxide layer 12 thereon and a doped region 14, the extent of which within semiconductor material 10 being shown by dashed line 16. A capacitive plate 18 is bonded to the oxide 12, and an electrode 20 is contracted to plate 18. Semiconductor material 10 is also grounded by way of a second electrode 24.
Ports A and B, of electrodes 20 and 24, respectively, are connected to a C-V meter. C-V measurements are made according to the technique, for example, of Nicollian and Brews, "MOS Physics and Technology", (Wiley, New York, 1982), p. 383, and the average dopant level underneath the plate 18 is derived. Typical dimensions for this kind of measurement are an oxide 12 thickness of approximately 20 nm, a plate 18 thickness of approximately 500 nm, a semiconductor material 10 thickness of approximately one millimeter, and an average dopant region 14 depth of approximately 1000 nm. The lateral extent of plate 18 is typically approximately of the order of a millimeter.
The limitations of the technique are readily apparent. Dashed line 16 in FIG. 1 represents the variations in the depth of dopant region 14. This region 14 has a varying depth, determined, for example, by boundaries between semiconductor devices within an integrated circuit. The measurement provided by the above scheme only provides gross average information about the vertical dopant density, and reveals nothing about lateral dopant profile in such cases.
In addition, though the lateral extent of dopant is of equal importance in device design to dopant depth, heretofore it could only be inferred through clever experimentation and extrapolation of 1D measurements, or, after the fact, through device performance. See for example P.M. Fahey, P.B. Griffin and J.D. Plummer, Rev. Mod. Phys. Vol. 61, 289 (1989), and E.H. Nicollian and J.R. Brews, above.